Of the various components in the human eye, the cornea is the principal optical element for refracting incident light onto the retina in the form of a clear image. Photorefractive keratectomy (PRK) is a procedure which typically utilizes an excimer laser beam to vaporize "ablate" corneal tissue in a precise manner to correct for focussing deficiencies of the eye. An excimer laser is preferred for this procedure because pulsed ultraviolet ablation is predictable, discrete, and non-damaging to adjacent tissue. PRK generally involves mechanical removal of the epithelium or outer layer of the cornea to expose the Bowman's layer on the anterior surface of the stroma. Laser ablation usually begins at the Bowman's layer. The laser beam removes corneal tissue to varying depths as necessary for recontouring the anterior stroma. Afterward, the epithelium rapidly regrows and resurfaces the contoured area, resulting in an optically correct (or much more nearly so) cornea. In a variation of the procedure, a surface flap of the cornea is folded aside and the exposed surface of the cornea's stroma is ablated to the desired surface shape with the surface flap then being replaced.
The specific region of the cornea involved in the refractive image formation will vary with the size of the pupil. Only a small central corneal region will refract light onto the cornea when the pupil is constricted under bright lighting conditions. Under dim lighting conditions, when the pupil is substantially dilated, a much larger corneal region is involved forming an image on the retina. This variation in pupil size can become an issue for a PRK patient if the diameter of the laser-treated corneal region ("the optical zone") is smaller than the dilated pupil diameter. When the ablated optical zone is smaller than the patient's dark adapted pupil size, the patient's night vision is affected. Typically, the patient's vision will be hazy or somewhat blurred, and the patient may perceive halos around bright lights. Approximately 20 percent of patients treated with a 5 mm optical zone have complained of such problems. This is a result of the pupil becoming larger than 5 mm as the pupil adapts for darkness. When a 6 mm optical zone is ablated, it is estimated that only 2 percent of patients complain of night-vision problems.
One apparent strategy for avoiding such night-vision problems would be to treat all patients with an optical zone larger, much larger, than the maximum pupil diameter. However, there are several disadvantages to this approach. First, maximum pupil diameter varies from patient to patient. Second, the maximum depth of laser ablation and the total volume of tissue removed both increase with optical zone diameter. Such increases typically lead to more regression, that is, deterioration, of the refractive change as the cornea heals. In addition, the increased tissue volume to be removed necessitates a longer laser procedure. Variations in corneal ablation behavior over time due to hydration changes in the de-epithelialized tissue (known to occur) maybe degrade the accuracy of the ablative corneal reshaping for lengthy procedures. A much more desirable strategy, not advanced until the present invention, is to tailor the optical zone diameter in each treatment to the maximum pupil diameter of that patient.